Direct access storage devices (DASD) have become part of every day life, and as such, expectations and demands continually increase for greater speed for manipulating data and for holding larger amounts of data. To meet these demands for increased performance, the mechanical assembly in a DASD device, specifically the Hard Disk Drive (HDD) has undergone many changes.
In operation, the hard disk is rotated at a set speed via a spindle motor assembly having a central drive hub. Additionally, there are tracks evenly spaced at known intervals across the disk. When a request for a read of a specific portion or track is received, the hard disk aligns the head, via the arm, over the specific track location and the head reads the information from the disk. In the same manner, when a request for a write of a specific portion or track is received, the hard disk aligns the head, via the arm, over the specific track location and the head writes the information to the disk.
Areal densities of hard disk drives (HDD) in the past have increased at significant rates of 60 percent to more than 100 percent per year. Although this trend has slowed in the past few years to approximately 40 percent per year due to technology challenges introduction of perpendicular recording has again increased this growth rate. Areal densities today are close to 250 Gb/in2. HDDs are being used more often as digital applications in the consumer electronics industry proliferates, requiring much higher capacities and setting new expectation for lower acoustics. All of the above makes fluid dynamic bearing spindle motors attractive for minimizing non repeatable run-out (NRRO), lowering acoustical noise, and improving reliability.
Presently, the transition from ball bearing (BB) spindle motors to fluid dynamic bearings (FDB) has completed in the HDD industry. In general, by incorporating FDB motors in HDD designs higher areal densities and much faster spindle speeds are achieved for today's applications. For example, NRRO is the highest contributor to track mis-registration (TMR), thus impacting HDD performance. NRRO is also an inhibitor in achieving higher track densities. Ball bearing motors produce larger NRRO due to the mechanical contact with the inherent defects found in the geometry of the race ball interface and the lubricant film. Ball bearing spindle motors have minimized this issue with tighter tolerances and closer inspections. There is an upper limit beyond which the ball bearing design can no longer overcome the NRRO problem at the higher areal densities. Currently with ball bearings, NRRO has settled in the 0.1 micro-inch range.
By contrast, FDBs generate less NRRO due to absence of contact between the sleeve and stator. FDB designs are expected to limit NRRO in the range of 0.01 micro-inch. Other inherent properties of the FDB design are higher damping, reduced resonance, better non-operational shock resistance, greater speed control, and improved acoustics. Non-operational shock improvement is a result of a much larger area of surface-to-surface contact. Noise levels are reduced to approximately 20 dBA, since there is no contributing noise from ball bearings.
However, problems with FDBs are the contamination of the head disk enclosure with the lubrication and loss of fluid within the bearing. For example, significant oil loss is observed from server class fluid bearing disk drive motors during accelerated life tests at elevated temperature. The pathway for oil loss from the upper part of a bearing in one design type is schematically illustrated in FIG. 1. FIG. 1 depicts a sectional view of an upper motor bearing 11 showing the oil 13, the pathway 15 for oil loss through a seal gap 17, and the seal 19 that slows down the rate at which oil leaves the bearing cavity 21. Sealing the motor bearing 11 decreases the oil loss, but oil is still emitted through the required air gap 17 in the seal 19.
For example, the oil loss from a bearing during accelerated motor reliability testing is shown in FIG. 2. FIG. 2 illustrates the oil remaining in a bearing as a function of time during continuous running at elevated temperature. The bearing was tested with a seal (upper line 25) and without a seal (lower line 27).
As shown in FIG. 3, the oil 13 exits the bearing 11 by first passing across the oil-air interface 31 into the bearing cavity 21, possibly via oil-air interface instability. The oil then convects and diffuses (e.g., indicted by arrows 15) as oil mist 33 and/or oil vapor 35 out of the interstitial region of the bearing cavity 21 through the seal gap 17. As demonstrated in FIG. 2, the oil loss from the cavity can be reduced with an improved seal gap configuration. Oil loss also can be decreased through the use of a less volatile oil. However, that option is a less desirable solution since oil with lower volatility has a higher viscosity that can result in a 30% increase in power consumption and difficulty in cold starts. Thus, an improved solution would be desirable.
It is known that hydrocarbon monolayer's inhibit vaporization of water by as much as 40 or 50% in applications involving cooling water in evaporative towers. See, e.g., U.S. Pat. Nos. 4,099,915 and 4,147,658. Hydrocarbon surfactants cannot readily form a low surface tension monolayer on the surface of another hydrocarbon, so these methods have not been successfully applied to suppress oil evaporation. Since evaporation of volatile hydrocarbons such as solvents and gasoline is a recognized problem, low permeability aqueous foam has been used to cover those types of surfaces. See, e.g., U.S. Pat. Nos. 5,434,192 and 5,296,164. However, a foam will not work in the fluid bearing motor of a disk drive because of the very small annular space limitations and because foams are not stable for the long life required of magnetic storage devices. U.S. Pat. No. 5,935,276 discloses another solution that uses a soluble polymer film to inhibit the evaporation of solvents or gasoline. Although polymers increase the viscosity of the fluid bearing oil, it does not provide enough suppression of evaporation.
Thus, none of the known solutions for suppressing the evaporation of fuel or oil are sufficient to provide an integral evaporation barrier in the presence of interfacial shear flow that is present on the oil meniscus in a disk drive fluid bearing spindle motor. In contrast, the invention disclosed herein teaches that a novel immiscible fluid forms a layer on oil that prohibits the evaporation of oil, and remains intact in the presence of surface shear flow of a disk drive motor.